Steam boiler comprising a radiation element

ABSTRACT

A steam boiler has at least one water and/or steam conveying element and at least one radiation element, which is an un-cooled element, arranged in the flow of hot flue gases, such that it is convectively heated by the flue gases. The radiation element is located at a pre-determined distance from the at least one water and/or steam conveying element, wherein the pre-determined distance is arranged such that the flow of hot flue gases between the radiation element and the water and/or steam conveying element is unhindered, and such that the water and/or steam conveying element is heated by heat radiation from the radiation element.

TECHNICAL FIELD

The present invention relates to a steam boiler according to the preamble of claim 1.

BACKGROUND

Steam boiler power plants are provided with water and/or steam conveying elements such as steam tubes in the boiler walls and also often superheaters for converting the saturated steam that is produced in the boiler to dry superheated steam which is efficient for power generation.

The overall operation principle of a boiler that produce hot water or steam is the generation of hot flue gases by burning a fossil or a renewable fuel to generate hot water or steam that in case of a power plant is converted into electricity. The hot gas transfers the thermal energy via heat exchangers to hot water or pressurized steam that may be used for domestic and industrial heating, in industrial processes, or in a power boiler to generate electricity. The final stage heat transfer in the latter case is normally done in superheaters that typically consist of bundles of tubes arranged in the power boiler. In operation, the hot water or steam that is produced in the boiler wall tubing is redirected back into the boiler through the superheater tubes. The hot flue gases from the boiler heats the superheater tubes mainly convectively and the heat is conducted through the superheater tube walls to the steam that flows therein. The temperature of the steam is thereby increased so that superheated dry steam is produced.

Hence, in conventional boilers, the heat transfer from the flue gas to the boiler walls or the superheaters is restricted to mainly convective heating from the hot flue gases in the boiler. Therefore, the boiler walls and the superheaters must have a large surface area in order to achieve an effective heat transfer from the hot flue gases to the water or steam. As a consequence thereof, conventional boilers and superheaters are associated with the problem of high material costs. A further problem is that the voluminous boiler walls and superheaters increase the total size of the power plant with resulting high construction costs for the power plant.

One example of such a boiler is shown in U.S. Pat. No. 4,325,328 which shows a steam boiler which is constructed from interconnected tubes defining four enclosing walls and a partition, also manufactured from interconnected tubes, which is positioned within the enclosing walls such that two combustion chambers are defined. Water is evaporated into steam in the enclosing walls and the partition. The steam is thereafter conducted through a platen heating surface for first superheating and then through a further superheater.

It is also known to arrange flue gas deflectors and baffles to control the gas flow in boilers.

U.S. Pat. No. 4,226,279 shows fins that are welded to tubes in a steam boiler in order to prevent soot particles from moving laterally over the steam tubes. U.S. Pat. No. 4,226,279 is directed to overcoming the problem of warping and buckling of baffle plates in boilers. This problem is solved in U.S. Pat. No. 4,226,279 by welding the fins directly to the tube sections so that the fins are cooled by the fluid that that flows through the tube.

GB10233 is aiming at solving the problem of achieving a uniform gas flow over the boiler tubes. According to GB10233 this is achieved by introducing baffle plates that presents a resistance to the gas flow on some positions in the boiler and therefore forces the gas to flow more uniform over the boiler tubes. In GB10233 the baffle plates are in contact with the water tube or, alternatively the baffles plates are arranged perpendicular to the gas flow. This restricts the gas flow over the baffle plates and as a consequence thereof the baffle plates do not assume a temperature much higher than the water tube.

JP49104001 shows baffle plates that are arranged in a steep angle to the gas flow in order to collect soot particles in a portion of a boiler.

Further attempts have also been made to increase the heat transfer to the superheater tubes and to increase combustion efficiency. One such commercialized design is the circulating fluidized bed boiler in which a heat transfer of 250-300 kW/m²° C. is achieved by circulating hot solid particles (sand and ash) in the boiler volume and also around the superheater tubes. However, this method is related with higher complexity and cost of the boiler and increased wear on the superheater tubes and other parts in the interior of the boiler.

A similar method is described in FR1154090 which describes a boiler in which vapour flows through a plate shaped superheater and is heated by heat radiation from hot soot particles in the flue gases.

Hence, it is an object of the present invention to achieve an energy efficient boiler. A further object of the present invention is to achieve a cost effective steam boiler. Yet a further object is to achieve a boiler in which emissions in the form of NOx-gases and unburnt hydrocarbons are reduced.

SUMMARY OF THE INVENTION

According to the invention, at least one of these objects is achieved by a steam boiler comprising at least one water and/or steam conveying element that is heated by the hot flue gases in the boiler characterized in that the steam boiler comprises at least one radiation element wherein; said radiation element is an un-cooled element; said radiation element is arranged in the flow of hot flue gases such that it is convectively heated by the flue gases; said radiation element is located at a pre-determined distance from said at least one water and/or steam conveying element such that the flow of hot flue gases between the radiation element and the water and/or steam conveying element is unhindered, and such that the water and/or steam conveying element is heated by heat radiation from said radiation element.

The general principle behind the invention can be explained as follows: In operation of the steam boiler, both the water and/or steam conveying tubes and the radiation element are heated convectively by hot flue gases and consequently both the water and/or steam conveying element and the radiation element will, when geometrical assumptions are applied, radiate heat energy in accordance with Stefan-Boltzman Law:

P=φ·σ·ε·(T _(e) ⁴ −T _(s) ⁴)

where P is radiated energy per surface area of the radiation element in W/m² φ is a geometry factor depending on the amount of self screening of the radiation element between 0 and 1

σ is 5.67 10⁻⁸ W/m²

ε is the surface emissivity (typically about 0.7 to 0.9 for oxidized surfaces) and T_(e) and T_(s) are temperature of radiation element and surrounding in ° K

However, the difference between radiation element and the tubing is that the water and/or steam conveying tubes are cooled by the fluid that flows there trough whereas the radiation element is an un-cooled element. Hence, no steam or water flows or circulates in the radiation element

Since the radiation element is situated in the hot flue gas and not cooled by steam it will be heated to a much higher temperature than the steam conveying tubes.

In operation of a steam boiler, even the temperature of the hottest components in contact with steam or water, namely the temperature of the outer surfaces of the steam conveying tubes, is typically not more than 50° C. higher than the maximum temperature of the steam that flows through the tube, which typically is not higher than 400-650° C. The radiation element on the other hand assumes a temperature closer to that of the flue gases which typically is between 800-1250° C. The radiation element will therefore, when the temperatures have reached a steady state, radiate more heat than the steam conveying tubes, typically in the order of 3 to 10 times more heat per surface area. This heat is largely absorbed by the steam conveying tubes.

Due to the radiation element, the total heat transfer to the superheater tubes and/or the boiler walls is increased in comparison to conventional superheaters that mainly are heated by convection. Due to the increased heat transfer per unit area, the size of the boiler wall and superheaters can be reduced with maintained efficiency of the boiler.

The invention also provides further advantages.

Emissions in the form of NOx-gases and unburnt hydrocarbons are reduced in the flue gases since the surfaces of the radiation elements and the water and/or steam conveying elements in the boiler are heated to a temperature at which gas/surface reactions are kinetically promoted. Hence, due to the high temperature of the radiation element, more chemically active surfaces are provided where the flue gases can react to more stable compounds.

In conventional boilers, the flue gases condense on the relatively cool surfaces of the tubing in the boiler and form large amounts of deposits. In the inventive boiler, these problems are minimized since the heat radiation from the radiation elements provides a possibility to control the location of deposit condensation to regions in the boiler where the consequences are smaller. It is further believed that the introduction of heat radiation on the surface of condensed deposits may alter or even dissolve the condensed coatings. Increased radiation on one portion of e.g. the steam tubes in the boiler wall may raise the surface temperature to a level where non-desirable compounds will not condense.

In the present invention it is important that the radiation elements are arranged such that the flue gases flow unhindered over the radiation element, over the steam conveying tubes and also flow unhindered in the space between the radiation elements and the steam conveying tubes. Unhindered flow of flue gas is important to ensure that as much heat as possible is transferred by convection from the hot flue gases to the steam conveying tubes. Unhindered flow over the radiation element is also important in order to ensure that the convective heating of the radiation element is maximized, because the higher the temperature that the radiation element assumes, the more heat energy will be transferred by heat radiation from the radiation element to the steam conveying tubes.

By flowing “unhindered” is meant that the flue gas flows freely over the surfaces of the radiation element and the surfaces of the steam conveying tubes and that the flow of flue gas is not restricted in any way between the radiation element and the steam conveying element. Hence, the radiation element should be arranged at a pre-determined distance from the steam conveying tubes, i.e. such that there is an open space between the radiation element and the steam conveying tubes.

The pre-determined distance between said at least one radiation element and said at least one steam and/or water conveying element is therefore preferably adopted such that an unhindered flow of flue gases is permitted between said radiation element and said steam and/or water conveying element. Preferably, the pre-determined distance between the radiation element and water and/or steam conveying element is large enough to guarantee that the flow of the flue gases between the latter is large enough to generate a positive heating effect of the flue gases on the water and/or steam conveying element. The pre-determined distance should be optimised for each specific case, depending on pressure conditions in the boiler, flue gas temperature etc. It is preferred that the radiation element is not too distant from the water and/or steam conveying element, in order to provide for a positive heating effect of said radiation element. A maximum distance of approximately 500 cm, preferably 250 cm, more preferably 100 cm, even more preferably 60 cm, and most preferably 30 cm, is conceived for most applications.

According to the invention a minimum distance between the radiation element and the water and/or steam conveying element is 20 cm, preferably 10 cm, preferably 5 cm, more preferably 1 cm, more preferably 2 cm, even more preferably 5 mm, most preferably 3 mm. Thereby, a sufficient flow of flue gases is achieved for most conceivable boiler applications. The spacing is open and will allow flow of flue gases between said elements.

In order to ensure maximal convective heating of the radiation element it is also important that the radiation element is arranged in the flow of flue gas such that all sides of the radiation element are exposed to the flow of flue gas, i.e. there should be a flow of flue gas on all sides of the radiation element. The radiation element should therefore be arranged at a pre-determined distance from other parts of the boiler, for example the circumferential walls such that there is an open space between all side surfaces of the radiation element and other parts of the boiler. Preferable, the radiation element is arranged at a distance of at least 5 mm, preferably at least 1 cm, more preferably at least 5 cm, even more preferably at least 10 cm from boiler parts which not form part of the radiation element.

To maximize the convective heat transfer to the steam conveying tubes it is also important that the radiation element does not deflect the flow of flue gas away from the steam conveying tubes. Hence, the radiation element should be arranged such that the flue gas can flow past the radiation element in a direction towards the steam conveying element without essentially changing the main flow direction. Thus, the radiation element should be arranged such that the flue gases can flow in a constant flow direction over the radiation element.

The “main flow direction” is the flow direction from the burner section towards a gas outlet in the boiler, or from a gas inlet towards an gas outlet.

To avoid deflection of the gas flow, the radiation element is preferably arranged such that it extends in a direction which is essentially parallel to the main flow direction of the flue gases.

According to a first embodiment the radiation element is sheet, such as a flat steel sheet. An advantage with a flat sheet is that it has a large surface area in relation to its weight. This is advantageous for the radiation efficiency. The steel sheet should be arranged such that a normal to its large side surfaces is perpendicular to the direction of the gas flow, i.e. that the side surfaces of the steel sheet are parallel with the flow direction. The steel sheet should further be arranged such that its edge portion, which is relatively narrow and therefore the presents little resistance to the gas flow, faces the gas flow.

It is also possible that the radiation element is a corrugated steel sheet, i.e. meander shaped. An advantage with a corrugated sheet is that it is very rigid.

The radiation element may also be a flat or a corrugated strip (typically 1-20 cm wide). The radiation element should be as thin as possible in order to minimize the weight. However, to ensure thermal stability and to avoid rapid failure due to corrosion, the thickness radiation element should at least be 0.5 mm. Typically, the thickness of the radiation element is 0.5-20 mm, preferably 1.5-10 mm. The length and height of the radiation element is selected in dependency of the application in question.

According to a second embodiment the radiation element is an elongated rod element. The rod element could have circular cross-section, such as a round bar or a wire or a thick walled tube. It could also have rectangular cross-section. The rod shaped radiation element should be arranged such that the longitudinal axis of the rod element is parallel to the gas flow and such that the normal to the longitudinal axis of the rod element is perpendicular to the gas flow.

It should be appreciated that the radiation element is a separate, individual element which deliberately is arranged in the boiler in order to increase the efficiency of the boiler by radiating heat to the steam and/or water conveying elements.

In order to maximize the convective heating of the radiation element it is preferred that the radiation element extends in the flow of flue gases from an upstream side of the steam conveying tubes towards a downstream side thereof. In a superheater arrangement it is preferred if the radiation element is arranged in an upstream portion of the superheater arrangement. The reason for this is that the flue gases are cooled as they flow over the steam conveying tubes. To expose the radiation element to the hottest gas it should preferably be arranged as far upstream as possible in relation to the steam conveying tubes. If the radiation element is arranged in a downstream portion of a superheater arrangement, it will be exposed to flue gases of lower temperature and the convective heating of the radiation element will not be effective.

The radiation element should preferably be attached to a surface in the boiler which has a temperature as close as possible to the radiation element, preferably the same temperature. Thereby are temperature gradients avoided in the radiation element which could lead to mechanical stress and buckling and cracking of the radiation element. Therefore, it is suitable to attach the radiation element to a portion of the boiler roof or wall that is not covered with steam tubes.

Preferably, the radiation element is flexibly, i.e. movably, attached to the boiler so that the radiation element can move as it is subjected to thermal expansion during heating. The advantage thereof is that the build up of mechanical stress is avoided in the radiation element.

Preferably, the radiation element comprises fastening elements in the form of hooks or rings so that the radiation element may be hung onto e.g. a bar in the boiler. The hooks or rings allow the radiation element to move during thermal expansion.

When the radiation element comprises hooks or rings it may also be hung directly onto a steam and/or water conveying element, such as a superheater tube. The hooks or rings allow the radiation element to move and thereby is mechanical stress due to temperature differences between radiation element and steam and/or water conveying element avoided. Preferably, the fastening elements are manufactured from thin wires (e.g. 1-5 mm thick) in order to minimise heat transfer between radiation element and the steam and/or water conveying elements.

The function of the fastening elements hooks is purely mechanical and does not contribute to the radiation function.

Preferably, the radiation element is attached to the boiler such that at least one end of the radiation element is free. Thereby, expansion of the radiation element, normally heat expansion and creep elongation, is permitted.

According to an alternative the water and/or steam conveying element is at least one steam conveying superheater tube.

According to an alternative said water and/or steam conveying element is the water and/or steam conveying tubes in at least a portion of the wall of the boiler.

According to an alternative said water and/or steam conveying element is a double walled lining.

Preferably, the surface area of the portion of the radiation element that faces said at least one water and/or steam conveying element is at least 3 percent of the total outer surface area of said at least one water and/or steam conveying element.

According to one alternative said radiation element comprises a concave surface which is turned towards an adjacent water and/or steam conveying element. Thereby, the radiation from the radiation element is focused on the water and/or steam conveying element, in particular if the latter has a tubular shape and is thereby partly enclosed by the radiation element. Preferably, the concave surface is a surface of a bent sheet forming said radiation element. Preferably, the radius of said concave surface is within the range of 0.8-2 times, preferably 1.0-1.5 times the radius of the water and/or steam conveying element in the case that the latter is tubular.

Preferably, the radiation element is formed by an alloy which is based on Fe or Ni and contains Al and which, when subjected to heat in an oxygen-containing atmosphere, forms a protective alumina layer on the outer surface thereof. Such steel has the advantage of having a superior heat resistance and presenting a long life as radiation elements in the harsh environment generated in a boiler.

According to a particularly preferred embodiment, the radiation element is formed by steel that contains 10-30 mass %, preferably 15-25 mass % Cr, 2-7 mass % Al, balance Fe and unavoidable impurities. Such steel has excellent heat resistance, corrosion resistance and ability of generating protective alumina layer when subjected to heat in an oxygen-containing atmosphere. Preferably, the steel should be subjected to temperatures of 700° C., preferably 1050° C. or above in order to obtain such a protective alumina layer.

According to yet another embodiment the radiation element is formed by steel that contains 10-30 mass %, preferably 15-25 mass % Cr, 2-7 mass % Al, 1-4 mass % Mo, balance Fe and unavoidable impurities. The presence of Mo in this steel contributes to a further improved hot strength.

According to yet another embodiment the radiation element is formed by steel that contains 10-30 mass %, preferably 15-25 mass % Cr, 2-7 mass % Al, 1-4 mass % Mo, 0.01-1.0 mass % rare earth metals (REM), balance Fe and unavoidable impurities. REM contributes to an improved corrosion and oxidation resistance.

According to yet another embodiment the radiation element is formed by steel that contains 10-30 mass %, preferably 15-25 mass % Cr, 2-7 mass % Al, 1-4 mass % Mo, 0.01-1.0 mass % rare earth metals (REM), 0.05-2.0 mass % Ti, Zr, Y and Hf, balance Fe and unavoidable impurities. REM contributes to an improved corrosion and oxidation resistance.

The steam boiler may comprise a plurality of radiation elements. In a boiler design in which the water and/or steam conveying element is arranged in one or more rows, the radiation elements may then be located between such rows or on each side of each row. Thereby, each such row may be heated from two opposite sides thereof by two adjacent radiation elements arranged on opposite side of said row.

The radiation elements may be distributed in a predetermined manner such that they cover predetermined parts of the water and/or steam conveying element. Thereby, the radiation element will have the technical effect of enabling a control of the condensation of depositions.

DESCRIPTION OF DRAWINGS

FIG. 1: A schematic illustration of a steam boiler plant according to a first preferred embodiment of the invention.

FIG. 2: A schematic illustration of a section of an arrangement of superheater tubes in the inventive steam boiler.

FIG. 3: A schematic illustration of an arrangement of radiation elements and superheater in a steam boiler according to a first preferred embodiment of the invention.

FIG. 4: A schematic illustration in side view of an arrangement of radiation elements and superheater in a steam boiler according to a second preferred embodiment of the invention.

FIG. 5: A schematic illustration in top view of an arrangement of radiation elements and superheater in a steam boiler according to a second preferred embodiment of the invention.

FIG. 6: a schematic illustration in top view of a conventional superheater arrangement forming basis for a heat transfer calculation.

FIG. 7: a schematic illustration in top view of an inventive superheater arrangement forming basis for a heat transfer calculation.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows schematically a steam boiler according to a first preferred embodiment of the invention. For clarity reasons only components that are relevant for the invention are shown.

The boiler 1 is a coal fired steam boiler. This type of boiler comprises a combustion zone 11 in which burners 11 produce hot flue gases of a temperature up to 1250° C. The temperature of the steam produced in the boiler is in the range of 400-700° C. The boiler could also be a bubbling fluidized bed steam boiler, in which the combustion takes place in a combustion zone of a one meter deep sand layer on the bottom of boiler.

The boiler 1 comprises a first section 10 and second section 20 which are defined by circumferential walls 9. It is possible that the boiler only comprises one section, i.e. the first section 10. It is also possible that the boiler comprises more than one two sections. Burners 11 are arranged in the combustion zone in the bottom portion 8 of the first section 10 of the boiler, in this case the burners are coal fired, however the burners could be fuelled by other types of combustive material such as natural gas. The burners 11 produce hot flue gases 12 which, under high turbulence, flow up through the first section of 10 of the boiler, over to the second section 20 and out through a gas outlet 40. In the case that the boiler only comprises one section, the gas outlet 40 is located in this section. The expelled flue gas 12 is thereafter subjected to catalytic purifying and released or used for other purposes. These steps are not shown in FIG. 1. The boiler 1 further comprises a roof 13.

The inner surfaces of the circumferential walls, i.e. the surfaces that faces the combustion gases, of the first and the second sections 10, 20 are lined with a water and/or steam conveying element in the form of steam tubes 30. In FIG. 1, only portions of the tubes 30 in the bottom of the boiler sections are shown in order not to obscure other relevant parts of the boiler. However, the tubes 30 run from the bottom of each boiler section 10, 20 to the top of each section 10, 20 so that essentially the whole interior of the boiler is covered by tubes. Water enters the steam tubes at a water inlet 21 in the first boiler section 10 and is pumped through the boiler by a circulation pump, not shown. As the water is pumped through the steam tubes 30 from the first boiler section 10 the second boiler section 20 it is heated into steam by the hot fluid gases in the boiler.

The boiler also comprises further water and/or steam conveying elements in the form of two arrangements of superheater tubes for increasing the temperature of the steam coming from the steam tubes 30. In FIG. 1 a primary superheater arrangement 50 is arranged in the second section 20 of the boiler a secondary superheater arrangement 60 is arranged in the first section 10 of the boiler. However, it is obvious that any number of superheater arrangements could be arranged in the boiler.

Saturated steam is led from the steam tubes 30 in the second boiler section into the primary superheater 50. The steam is circulated through the first superheater arrangement 50 and led over to the secondary superheater arrangement 60 in the first section 10 of the boiler which comprises a steam outlet 52 where the superheated, dry steam is led out of the boiler.

The superheater arrangements 50 and 60 typically comprise several sections of U-shaped tubes 61 that are arranged in a side-by side manner so that a large tube volume is achieved. FIG. 2 shows a detailed view of a section 61 of a superheater arrangement that comprises several U-shaped tubes 61 a, 61 b, 61 c.

According to a first embodiment of the invention, the superheater arrangements 50, 60 comprise radiation elements 70 which, contrary to the superheater tubes or the steam tubes in the boiler, are not cooled by steam or water.

The radiation elements are arranged between the sections of U-shaped tubes 61 in the superheater so that as much as possible of the surface of the radiation elements faces the superheater tubes. In FIG. 1 the radiation elements are arranged parallel to the main flow direction of the flue gases. The radiation elements are partially obscured by the super heater arrangements 60 and 50.

FIG. 3 shows schematically in detail a radiation element and the three sections of the superheater arrangement 60.

For illustrative purposes, FIG. 3 shows three superheater sections 61 and one radiation element 70. However, it is obvious that the superheater arrangement could comprise any number of tubes sections 61 and also any number of radiation elements 70. For example, two additional radiation elements could be arranged in the empty spaces between the tube sections 61. The superheater sections 61 in FIG. 3 are identical to the superheater section 61 of FIG. 2, however, in order to not obscure the radiation element, only the outermost U-shaped tube is shown in each section.

The radiation element 70 is a flat sheet of heat resistant steel. A flat steel sheet is advantageous as radiation element since it is available at relatively low cost and covers a large surface. The flat steel sheet has two large flat side surfaces 71, 72 and a circumferential edge portion 73. Typically, the steel sheet has a thickness of 3-50 mm. As can be seen in FIG. 3, the radiation element 70 is arranged such that the normal N to its flat side surfaces is perpendicular to the main flow direction of the flue gas 12 and such that its edge portion 73 faces the gas flow.

Preferably, the steel is an alumina forming FeCrAl-steel which has high resistance to oxidation and corrosion from the flue gas. Preferably, the steel comprises, in wt %, 15 to 25% Cr, 2 to 7% Al, 1-4% Mo, 0.01-1.0% Rare earth metals, and balance Fe and unavoidable impurities. One such alloy is the commercially available alloy Kanthal APMT, commercialized by Sandvik AB. This alloy, which is dispersion strengthened through powder metallurgy, exhibits good corrosion properties, good mechanical strength and high resistance to creep deformation at high temperatures.

Another group of suitable alumina forming alloys are NiFeCrAl-alloys containing 15-30% Cr and 2 to 7% Al plus minor additions. Ni is balance but might also be partially substituted with Fe.

The radiation element is arranged so that at least one of its two large side surfaces faces the tube section 61 of the superheater. It is further dimensioned such that the total surface area of the portion of the radiation elements that faces the superheater tubes equals to at least 3 percent of the total outer surface area of the superheater tubes.

It has been shown that if the total surface area of the radiation elements is at least 3 percent of the total outer surface area of the superheater tubes a significant heat contribution by radiation is given to the steam in the superheater tubes. However, it is advantageous if the surface area of the radiation elements is large in comparison to the total outer surface area of the superheater tubes since the heat transfer by radiation to the superheater tubes thereby increases. Preferably, the surface area of the portion of the radiation elements that faces the superheater tube has a surface area that is at least 5 percent of the total outer surface area of the superheater tubes, more preferred 7 percent, more preferred at least 10 percent thereof, more preferred at least 15 percent thereof, more preferred at least 25 percent thereof.

The maximum dimensions of the radiation elements are limited by the flow conditions in the boiler as well as the operating conditions and design of the boiler and are determined in each separate case.

In the described embodiment, each of the radiation elements has a rectangular form with a height of 6 meters and a width of 2 meter. The radiation element may also be assembled from several smaller parts.

In order to not hinder the flow of flue gas around the superheaters, an opening could be provided in the radiation element. FIG. 3 indicates schematically with dashed lines the position of a rectangular opening 71 in the radiation element 70. The remained of the steel sheet, i.e. the border 72 around the opening covers the superheater tubes. Furthermore, the radiation element could be provided with turbulence promoting elements (not shown) to promote turbulent flow around superheater and radiation element.

The intensity per surface unit of the heat radiation from a point- or line shaped heat source decreases with the distance. In order to maximize the radiation exchange between the radiation element and the superheater it is therefore important, for a given geometrical size of the radiation element, that the distance between the radiation element and the superheater tube is as small as possible.

However, it is also important that the distance between the radiation element and the superheater tube large is large enough to allow the flue gases to flow unhindered over the superheater tube. Preferably, the distance should be large enough to allow a turbulent flow of flue gases between superheater tube and radiation element.

The radiation elements and the superheater tubes may have various shapes and dimensions and therefore the exact distance between superheater tubes and radiation elements must be determined for each application in question. In the preferred embodiment shown schematically in FIG. 3, the distance is between the radiation elements and superheater tube is 20 to 60 cm.

The radiation elements 70 are preferably attached to the roof of the boiler. According to one alternative, one or several steel bars 90 are attached in the roof of the boiler over the superheater tubes. The radiation elements 70 comprise fastening elements 80, for example pins, or hooks or rings that are attached, e.g. by welding or riveting to the upper edge of the sheet. The radiation element could comprises any number of fastening elements, for example two or three or five. The fastening elements are attached to the steel bar 90 so that the radiation elements hang down between the superheater tubes. This can be achieved in many different ways, the fastening elements may for example be welded to the bar so that the radiation element hangs in a fixed manner. It is also possible to attach the upper edge of the radiation element directly to the roof of the boiler. Also possible to attach the radiation elements to other portions of the boiler, for example the walls. However, in order to avoid buckling and warping, it is preferred that the radiation element is attached to un-cooled surfaces, i.e. surfaces which are not cooled by steam or water, for example a portion of the roof or to the bar 90.

Thermal expansion in combination with temperature gradients during operation of the boiler may introduce mechanical stresses in the radiation elements and cause deformation, such as bending or buckling. In order to prevent or reduce the mechanical stress in the radiation elements it is therefore preferred to arrange the radiation element so that at least one end thereof can expand freely, for example by hanging the radiation elements from the roof of the boiler as described above.

In order to achieve this, the radiation element comprises fastening elements in the form of rings or hooks and is hung onto the bar 90. This allows the radiation elements to expand in all directions and the amount of mechanical stresses is reduced even more.

When the radiation element comprises fastening elements in the form of rings or hooks it is also possible to hang the radiation element directly on to water and/or steam conveying elements in the boiler, for example onto a superheater tube.

It is also possible to arrange the radiation element so that it can be moved from the outside of the boiler towards or away from the superheater tube or so that the angle between radiation element and superheater tube can be changed from the outside the boiler. Accordingly, a radiation element displacement element may be provided, which is in engagement with the radiation element and extends to the outside of the boiler, such that it can be operated from outside the boiler for the purpose of displacing the radiation element 70. This can be achieved by attaching the steel bar 90, onto which the radiation element 70 is attached, to a pivotal tap in the roof of the boiler or by arranging the steel bar slidable in a slot in the roof. The steel bar 90 may be manoeuvred from the outside by a lever.

The function of the superheater arrangement should be clear from the forgoing. Thus, in operation, the flue gases 12 from the burners 11 heats the radiation elements 70 which at equilibrium assumes a temperature given by the flue gas temperature and the radiation heat loss. The net effect is that radiation heat is absorbed by the superheater tubes, which are cooler than the radiation elements, and further conducted to the steam that flows in the tubes.

It is also possible to arrange radiation elements adjacent to other water and/or steam conveying elements in the steam boiler.

According to a second embodiment (not shown), radiation elements are arranged adjacent the steam or water conducting tubes 30 that forms the walls of the boiler. Also in this case the surface area of the portion of the radiation elements that faces the steam tubes should be at least 3 percent of the total area of the steam tubes in the walls of the boilers in order to achieve a significant heat transfer to the steam or water in the tubes. However, depending on the design and dimension of the boiler a significant heat transfer could also be achieved when the surface area of the radiation elements is at least 3 percent of the total surface area of the tubes 30 in a portion of the walls of the boiler. For example, at least 3 percent of the total surface area of the tubes in one of the boiler sections 10, 20.

Preferably, the total surface area of the radiation elements should be at least 5 percent of the total outer surface area of the steam tubes, more preferred 7 percent, more preferred at least 10 percent thereof, more preferred at least 15 percent thereof, more preferred at least 25 percent thereof.

It is also possible that at least a portion of the boiler wall comprises a water and/or steam conveying element in the form of a double walled lining (not shown in the figures). This is typically an elongated rectangular closed space which is manufactured from steel sheets that are welded together. Water is introduced in one end of the double wall lining and in the other end the water is distributed over a manifold into the steam tubes that lines the boiler wall.

According to a third embodiment (not shown in the figures) radiation elements are arranged adjacent to such a double walled lining. Also in the case of a double walled lining, the surface area of the portion of the radiation element that faces the double wall lining should be at least 3 percent of the total area of the double wall lining to achieve a significant heat transfer. Preferably, the total surface area of the radiation elements should be at least 5 percent of the total outer surface area of the double walled lining, more preferred at least 10 percent thereof, more preferred at least 15 percent thereof, more preferred at least 25 percent thereof.

It is of course possible to arrange radiation elements both in the vicinity of the steam tubes in the wall of the boiler and in the vicinity of the superheaters and the double wall lining. It is also possible to only arrange radiation elements in the vicinity of some of these water and/or steam conveying elements. A selective application of the radiation elements provides the possibility to control the amount of heat flow in different portions of the boiler. It is thereby possible to compensate for a varying heat distribution that may be caused by fuels of varying composition or ash content.

The radiation elements could be spaced in any manner over the water and/or steam conveying elements. For example, several radiation elements could be arranged close to each other in one portion of the steam tubes in the wall of the boiler whereas other elements could be arranged further spaced apart in other portions of the boiler wall. As mentioned, it is thereby possible to control the amount and the location of depositions that condensate in the boiler.

According to a fourth embodiment, see FIG. 4, the radiation element is a rod shaped element in the form of an elongated bar of circular cross-section, such as a round bar. However, the radiation element could also have a rectangular cross-section. The radiation element could also be hollow, for example a thick-walled tube. The radiation element could have any suitable diameter, e.g. 2-20 mm and be of any length, depending on the size of the steam conveying element, for example 6 meters.

The advantage with radiation elements of round cross-section is that an equal amount of heat is radiated 360° around the radiation element. It is thereby possible to heat several steam conveying elements with relatively few radiation elements. The compact round radiation elements occupies little space and therefore have little impact of the gas flow.

The rod shaped radiation element is arranged in the flow of the flue gases so that flue gas meet the end surface of the radiation element. The radiation element is arranged such that its longitudinal axis L is parallel to the gas flow and its normal N is perpendicular to the gas flow.

EXAMPLES

The heating effect of the inventive radiation element in a steam boiler will in the following be shown by a calculated example. In the example, calculations of temperatures and heat transfers have been made based on empirical data from conventional boiler designs. Flue gas absorption and emission coefficient are assumed to be equal and all surfaces are assumed to have emission and absorption coefficient 0.8 and further to have the same convective heat transfer properties. For the calculation, primary radiation, first and second reflections and absorption in the gas volume has been considered.

The calculation shows the heat that is absorbed in a superheater arrangement in an oil fired compact boiler. One calculation is made for an inventive overheater arrangement with a radiation element and one calculation is made for a conventional overheater arrangement without radiation element.

FIG. 6 shows a conventional superheater tube arrangement in a side view. The gas flow is cross the tubes in the vertical direction. The superheater arrangement consists of several superheater tubes 60. The distance between the tubes is about 80 mm in this example. FIG. 7 shows an inventive superheater arrangement in which flat radiation elements 70 are arranged between the superheater tubes 60. Note that the radiation elements are arranged at a distance from the superheater tubes.

The input data and the results for the calculation are shown below in table 1.

The results shows that the total heat absorbed by the inventive superheater arrangement is increased by 19% in comparison to the conventional superheater arrangement, i.e. from 57 to 68 kW/m².

TABLE 1 Calculation showing the effect of a radiation element in combination with superheater tubes. Conventional Inventive superheater superheater arrangement with arrangement radiation element Radiation surface No Yes Gas velocity w_(g) 13.4 13.4 m/s Gas temperature t_(g) 1000 1000 ° C. Gas emission coefficient ε_(g) 0.07 0.07 — Chilled surface temperature t_(k) 480 480 ° C. Chilled surface emission coef ε_(k) 0.8 0.8 — Radiation surface emission ε_(s) 0.8 — coefficient Radiation surface temperature t_(s) 760 ° C. Heat absorbed by chilled surface per projected surface Convection q_(k) 49 49 kW/m² Radiation q_(s) 8 19 kW/m² Total q 57 68 kW/m²

Table 2 shows further simulation results of the present invention. In table 2 the temperature of water and/or steam conveying and the temperature of flat sheet radiation elements have been calculated for various flue gas temperatures and various boiler components.

TABLE 2 Relation between temperature of water and/or steam conveying surface, temperature of radiation element, and gas temperature. Cooled surface Radiation element Flue gas temperature temperature Temperature ° C. ° C. ° C. Boiler Tertiary 555 670 790 superheater Empty shaft 450 675 800 Panel superheater 490 710 900 Panel superheater 490 745 1000 Boiler surface 400 710 1000

Although particular embodiments have been disclosed in detail, this has been done for purpose of illustration only, and is not intended to be limiting. In particular it is contemplated that various substitutions, alterations and modifications may be made within the scope of the appended claims. For example, the radiation element may have any type of geometrical shape, such as the shape of an aeroplane wing or a spool shape. The boiler could also be of a type that only comprises steam and/or to water conveying elements in the form of steam/water tubes in the wall of the boiler. 

1. A steam boiler comprising: at least one water and/or steam conveying element heated by the hot flue gases in the boiler; and at least one radiation element, the at least one radiation element being an un-cooled element arranged in the flow of hot flue gases such that it is convectively heated by the flue gases, wherein the radiation element is located at a pre-determined distance from said at least one water and/or steam conveying element, the pre-determined distance being arranged such that the flow of hot flue gases between the at least one radiation element and the at least one water and/or steam conveying element is unhindered, and such that the at least one water and/or steam conveying element is heated by heat radiation from said at least one radiation element.
 2. The steam boiler according to claim 1, wherein said at least one radiation element is arranged such that the flow of the hot flue gases can pass said at least one radiation element essentially without changing flow direction.
 3. The steam boiler according to claim 1, wherein the at least one radiation element is arranged in the flow of flue gases such that the entire at least one radiation element is exposed to the flow of flue gases.
 4. The steam boiler according to claim 1, wherein the at least one water and/or steam conveying element is at least one steam conveying superheater tube.
 5. The steam boiler according to claim 1, wherein the surface area of the at least one radiation element is at least 3 percent of the area of said at least one water and/or steam conveying element that is directly exposed to the flue gases.
 6. The steam boiler according to claim 1, wherein the radiation element extends essentially parallel to the flow direction of the flue gases.
 7. The steam boiler according to claim 1, wherein said radiation element is a sheet having two side surfaces and a circumferential edge, wherein the at least one radiation element is arranged such that a normal to one of its side surfaces is perpendicular to the flow direction of the hot flue gases and such that an edge portion of the sheet faces the flow of hot flue gases.
 8. The steam boiler according to claim 1, wherein said at least one radiation element is a corrugated sheet.
 9. The steam boiler according to claim 1, wherein the at least one radiation element is an elongated rod element, wherein the at least one radiation element is arranged such that its radial axis R is perpendicular to the flow direction of the flue gases.
 10. The steam boiler according to claim 9, wherein the at least one radiation element has a circular cross section.
 11. The steam boiler according to claim 1, wherein the at least one radiation element is formed by an alloy which is based on Fe or Ni and contains Al and which, when subjected to heat in an oxygen-containing atmosphere, forms a protective alumina layer on the outer surface thereof.
 12. The steam boiler according to claim 1, wherein the at least one radiation element is flexibly attached to an un-cooled surface of the boiler.
 13. The steam boiler according to claim 1, wherein the at least one radiation element is flexibly attached to the boiler such that at least one end of the radiation element is free to expand or contract.
 14. The steam boiler according to claim 1, wherein the at least one radiation element is arranged such that it flexibly hangs from the at least one water and/or steam conveying element.
 15. The steam boiler according to claim 14, wherein the at least one radiation element includes hooks or rings for hanging the at least one radiation element on the at least one water and/or steam conveying element.
 16. The steam boiler according to claim 5, wherein the surface area of the at least one radiation element is at least 10 percent of the area of the at least one water and/or steam conveying element that is directly exposed to the flue gases.
 17. The steam boiler according to claim 9, wherein the at least one radiation element has a rectangular cross-section. 